1. Introduction and stratigraphical context
The Cotham Member of the Lilstock Formation (Penarth Group) (Figure 1) was deposited during the Rhaetian, a time of major environmental transition across Britain and Ireland. The Penarth Group records the shift from underlying hypersaline continental basin sedimentation (Mercia Mudstone Group) to overlying marine mudstones and limestones of the Lower Jurassic Lias Group (Warrington et al. Reference Warrington, Audley-Charles, Elliot, Evans, Ivimey-Cook, Kent, Robinson, Shotton and Taylor1980; Swift, Reference Swift1999; Benton et al. Reference Benton, Cook and Turner2002; Gallois, Reference Gallois2007, Reference Gallois2009). The Cotham Member is widely recognized in sections from Devon, north-eastward to Yorkshire and north-westward to Northern Ireland and in some offshore basins (Warrington et al. Reference Warrington, Audley-Charles, Elliot, Evans, Ivimey-Cook, Kent, Robinson, Shotton and Taylor1980; Tappin et al. Reference Tappin, Chadwick, Jackson, Wingfield and Smith1994; Warrington, Reference Warrington1997; Swift and Martill, Reference Swift, Martill, Swift and Martill1999; Raine et al. Reference Raine, Copestake, Simms and Boomer2020). Its thickness varies from approximately 0.5 m in Somerset (Whittaker and Green, Reference Whittaker and Green1983) to over 12 m in Warwickshire (Old et al. Reference Old, Sumbler and Ambrose1987), reaching 21.5 m in Leicestershire (Poole et al. Reference Poole, Williams and Haines1968).
Lithostratigraphy and facies recorded in key published sections through the Cotham Member showing the lithology and relative position of microbialite beds where present (excluding Devon). The inset map shows the section lines. Logs are arranged equally spaced irrespective of the true distance between sections.
Figure 1. Long description
Panel A: A stratigraphic column showing the lithostratigraphy and facies of the Cotham Member in the Lilstock Formation. The column is divided into different sections labeled with numbers and names, such as Kelston, Pylle Hill, and Aust Cliff. Various lithologies are represented by different colors and patterns, including limestone, claystone, siltstone, and sandstone. The positions of microbialite beds are also indicated. Panel B: A map showing the locations of the sections through the Cotham Member. The map includes labels for cities such as Gloucester, Bristol, and Cardiff, and shows the section lines connecting these locations. The inset map provides a broader geographical context.
Lithologically, the Cotham Member consists mainly of pale-coloured, fine-grained sediments – grey, green-grey or cream-weathering mudstones, calcareous mudstones and siltstones – with local limestones and sandstones. Marine macrofossils are largely absent other than in the lowermost and uppermost beds (Warrington et al. Reference Warrington, Audley-Charles, Elliot, Evans, Ivimey-Cook, Kent, Robinson, Shotton and Taylor1980). However, the unit is renowned for a distinctive microbialite bed, commonly known as the Cotham Marble, which has attracted attention since the mid-18th century (Owen, Reference Owen1754; Woodward, Reference Woodward1892; Thompson, Reference Thompson1894; Short, Reference Short1903; Hamilton, Reference Hamilton1961; Wright and Mayall, Reference Wright, Mayall and Monty1981; Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014). Traditionally considered a local feature of the Bristol area, the Cotham Marble has more recently been documented across a wider region from Devon to South Gloucestershire, suggesting a persistent, though discontinuous, occurrence near the top of the member (Gallois, Reference Gallois2007; Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014; Mayall and Wright, Reference Mayall and Wright2015).
The Cotham Member was deposited during a period of profound global ecological change associated with the end-Triassic mass extinction (ETME) (Raup and Sepkoski, Reference Raup and Sepkoski1982; Benton, Reference Benton1993; Whiteside et al. Reference Whiteside, Olsen, Eglinton, Brookfield and Sambrotto2010; Pálfy and Kocsis, Reference Pálfy and Kocsis2014; van de Schootbrugge and Wignall, Reference van de Schootbrugge and Wignall2016; Wignall and Atkinson, Reference Wignall and Atkinson2020; Petryshyn et al. Reference Petryshyn, Greene, Farnsworthe, Lunte, Kelley, Gammariello, Ibarra, Bottjer, Tripati and Corsetti2020; van de Schootbrugge et al. Reference van de Schootbrugge, van der Weijst, Hollaar, Vecoli, Strother, Kuhlmannd, Thein, Visscher, van Konijnenburg-van Cittert, Schobben, Sluijs and Lindström2020), volcanism of the Central Atlantic Magmatic Province (CAMP) (McHone, Reference McHone2003; Blackburn et al. Reference Blackburn, Olsen, Bowring, McLean, Kent, Puffer, McHone, Rasbury and Et-Touhami2013; Bond and Wignall, Reference Bond and Wignall2014; Davies et al. Reference Davies, Marzoli, Bertrand, Youbi, Ernesto and Schaltegger2017; Heimdal et al. Reference Heimdal, Svensen, Ramezani, Iyer, Pereira, Rodrigues, Jones and Callegaro2018; Korte et al. Reference Korte, Ruhl, Pálfy, Ullmann and Hesselbo2019; Ruhl et al. Reference Ruhl, Hesselbo, Al-Suwaidi, Jenkyns, Damborenea, Manceñido, Storm, Mather and Riccardi2020; Hounslow et al. Reference Hounslow, Posen, Warrington and Page2024) and rifting that led to the breakup of Pangea and formation of the proto-Atlantic (Hesselbo et al. Reference Hesselbo, Robinson and Surlyk2004; Schettino and Turco, Reference Schettino and Turco2009). Perturbations in the carbon isotope record suggest that the Cotham Marble stromatolites approximately coincide with an initial negative isotope excursion (Hesselbo et al. Reference Hesselbo, Robinson and Surlyk2004; Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2016). This association has prompted interpretations of the microbialites as ‘disaster forms’ colonizing a stressed post-extinction environment (Mander et al. Reference Mander, Twitchett and Benton2008; Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014, Reference Ibarra, Corsetti, Greene and Bottjer2015, Reference Ibarra, Corsetti, Greene and Bottjer2016), although this is debated (Mayall and Wright, Reference Mayall and Wright2015; Radley et al. Reference Radley, Twitchett, Mander and Cope2008; Fox et al. Reference Fox, Cui, Whiteside, Olsen, Summons and Grice2020, Reference Fox, Whiteside, Olsen and Grice2022).
While research focus on the Cotham Marble is clearly important, the Cotham Member also contains older microbialite horizons which provide further evidence on the fluctuating depositional conditions during the latest Triassic. One of these – the Estheria Bed – has long been known from the Severn Estuary (South Gloucestershire) area (Richardson, Reference Richardson1903 a) but has received very little detailed study.
2. Microbialites in the Cotham Member
Microbialites are commonly subdivided into four morphologically distinct types, based upon internal structure: stromatolites (laminated), thrombolites (clotted), leiolites (structureless) and dendrolites (dendritic) (Riding, Reference Riding and Riding1991). The Cotham Member of southwest Britain contains two microbialite horizons: the well-documented Cotham Marble near the top (Owen, Reference Owen1754; Hamilton, Reference Hamilton1961) and the Estheria Bed, approximately mid-member (Wright, Reference Wright1860; Richardson, Reference Richardson1903 a) (Figure 1). The latter exhibits similarities to the overlying Cotham Marble, but the microbialite component of the older Estheria Bed has received almost no attention. A bed closely resembling the Estheria Bed was mentioned briefly by Mayall (Reference Mayall1981, pp. 181–183), who described a clotted ‘thrombolitic’ stromatolite from the middle of the member at Culverhole, Devon, while Jones (Reference Jones1981, p. 86) identified the Estheria Bed at Aust and Wainlode cliffs as an ‘algal’ deposit, remarking that it contained features similar in aspects to the younger Cotham Marble when sectioned.
Both the Cotham Marble and the Estheria Bed microbialites share broadly similar morphologies, which has led to the beds being mistaken for each other (see discussion in Richardson, Reference Richardson1903 a). At Manor Farm Quarry near Aust, South Gloucestershire [British National Grid ST 5755 8970], both beds are observed in situ, confirming the presence of two distinct microbialite horizons (Figures 1 and 2). The Estheria Bed lies within an interval marked by soft sediment deformation that lies not far beneath a 2–3 cm ferruginous sand interpreted by Simms (Reference Simms2003) as a hiatal surface equivalent to the one recognized in the region by Mayall (Reference Mayall1983). In addition to this key locality, we also collected new ex situ Estheria Bed material from Gloucestershire at Garden Cliff at Westbury on Severn [SO 728 128], Sedbury Cliffs [ST 558 934] and Aust Cliff [ST 5648 8941]. The Cotham Marble was examined in situ, and samples were collected from known sites at Manor Farm Quarry and from the former Stowey Quarry, Somerset [ST 6130 5912] (Figures 1 and 3). In this study, specimens were also recovered from new localities in Worcestershire and Warwickshire (Figure 3).
Sedimentary log of the section at Manor Farm Quarry. The lowest mudstones are placed in the Westbury Formation following Simms (Reference Simms2007) rather than assigned to the Cotham Member as suggested by Radley and Carpenter (Reference Radley and Carpenter1998). The Langport Member of the Lilstock Formation is absent at the base of the Blue Lias Formation. An intra-Cotham Member hiatus recorded by Simms (Reference Simms2003) but not seen in this study due to weathering and vegetation of the face is recorded on the log.
Map showing the localities where microbialites have been recorded in the Cotham Member, along with new localities described in this paper. Pale orange = surface outcrop of the upper Triassic rocks (Mercia Mudstone Group and Penarth Group). New localities are denoted in the numbered key by *and existing localities with new material are denoted by †.
Figure 3. Long description
The map displays the geographic distribution of microbialites in the Cotham Member, highlighting specific localities across England, Wales, and Northern Ireland. The map includes surface outcrops of upper Triassic rocks, marked in pale orange, which encompass the Mercia Mudstone Group and Penarth Group. Key locations where microbialites have been recorded are numbered and labeled. New localities are denoted with an asterisk (*), and existing localities with new material are marked with a dagger (†). The map covers regions from Birmingham and Worcester in the north to Exeter in the south, and from Newport and Cardiff in the west to Bristol and Gloucester in the east. An inset map shows the location of microbialites in Northern Ireland, specifically around Belfast.
We reviewed published descriptions (outcrop localities and boreholes) of microbialite facies in the Cotham Member and plotted their distribution (Figure 3). The Cotham Marble in its classic ‘landscape’ form (interlayered laminated and dendritic microbialites) as opposed to simply laminated varieties has been well-documented from Gloucestershire (western England) to the south Devon coast, a geographic distribution of some 2000 km2 (Richardson, Reference Richardson1906b, Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014), despite some authors suggesting that the ‘landscape’ form is restricted to the Bristol area (Mayall and Wright, Reference Mayall and Wright2015). The geographical range of the Cotham Marble (‘landscape’) has been extended markedly by our discovery of reworked fragments in the base of the Langport Member at Southam Cement Works Quarry at Long Itchington, Warwickshire (SP 420 630, Radley, Reference Radley2002) and in situ discovery at the nearby Ufton landfill site [SP 3932 6165] (Figure 3). Specimens from these locations include the ‘landscape’ variety. New specimens from Worcestershire comprise fragments of landscape Cotham Marble from ploughed fields at [SO 927 597] and roadside ditches [SO 943 593] around the villages of Phepson and Saleway near Droitwich Spa, Worcestershire (Figure 3). Numerous specimens were also collected in 2009 from a dump of Cotham Member material in a disused quarry along Trench Lane, Saleway [SO 928 595], with material having been excavated from a nearby building site. Including the new localities, 99 locations have yielded the Cotham Marble microbialite across Warwickshire, Worcestershire, Gloucestershire, Somerset and Bristol and further afield in Devon (Figure 1, Table S1 online Supplementary Material at http://journals.cambridge.org/geo). Specimens figured in this study are housed in the collections of the Lapworth Museum, University of Birmingham.
At eight locations, mostly in Somerset, but also in Gloucestershire and Devon, a ‘Cotham Marble equivalent’ of uncertain affinity has been recorded by early authors. Three sites in Gloucestershire and Somerset have microbial bioherms containing abundant Microtubus microfossils. The Estheria Bed microbialite is recorded from nineteen locations, and six localities have beds near the top of the Cotham Member described previously as ‘algal’ or are microbialites that do not resemble either the Estheria Bed or the Cotham Marble (Figure 2). Included in this category are stromatolites collected from the top of the Cotham Member at Cloghan Point, Northern Ireland [Irish Grid J4695 9066].
2.a. Estheria Bed microbialite
The Estheria Bed (Wright, Reference Wright1860) is one of the few limestones recognized in the Cotham Member (Figure 1). It was first recorded as the ‘Cypris- and Plant-Bed’ (Brodie, Reference Brodie1845, p. 61) and later renamed after the small branchiopod crustaceans that it often contains (the spinicaudatan Euestheria). In subsequent work, Richardson (Reference Richardson1903 a) described four Estheria Bed carbonate lithologies: laminated, nodular, shelly and those with fabrics analogous to the Cotham Marble. Here, we use the term ‘arborescent’ fabric to identify 19 Estheria Bed localities with microbial facies (Figure 3; Table S1 online Supplementary Material at https://journals.cambridge.org/geo). These extend from Devon (Wright, Reference Wright1864; Richardson, Reference Richardson1906 a) to Gwent (Richardson, Reference Richardson1905 a), Gloucestershire (Richardson, Reference Richardson1903 c, Reference Richardson1904 a, Reference Richardson1906 b) and south Worcestershire (Richardson, Reference Richardson1904 b).
The new Estheria Bed microbialite specimens all comprise a clotted-textured microbialite forming discontinuous bioherms 10 to 40 cm in diameter. At Westbury on Severn, Aust Cliff and Manor Farm Quarry, the bed contains conical to domal bioherms 2.5–4.5 cm high and 10–15 cm in diameter (Figures 4 and 5), typically comprising a basal unit of non-laminated to mottled micritic mudstone and laminated carbonate grainstone (Figures 4 and 5a–c). The proportion of grainstone to mudstone is variable, and the bioherms locally sit directly on top of mudstone, separated by an irregular boundary with only a centimetre of relief (Figure 4d). Larger bioherms (up to 40 cm diameter and 18 cm high) at Sedbury Cliffs rest on grainstone and flake breccia composed of thrombolitic and stromatolitic microbialite clasts (Figure 6). In most samples, the grainstone is overlain by calcareous mudstone and several millimetre-thick layers with stromatolitic lamination (Figures 5b–c and 6) over which the thrombolites lie.
Megastructural and macrostructural features of the Estheria Bed microbialite from Westbury on Severn and Aust. (a) External surface of the bed shown in (d), Westbury on Severn. (b–d) Conical and domal specimens from Westbury on Severn. (e) Estheria Bed microbialite from Aust.
Macrostructural and mesostructural features of the Estheria Bed microbialite. Images a–e and g–h are from Westbury on Severn; (f) is from Aust Cliff: scale bars = 2 mm diameter. (a) Grainstone facies from the base of the bioherm. (b) Disrupted stromatolite laminae at the base of the thrombolite bioherm. (c) Horizontal thrombolites passing upwards into vertically oriented forms. (d) Vertical thrombolites with intervening grainstone and rudstone. (e) Well-defined thrombolite with spar-filled vugs. (f) Coalesced thrombolites with a clotted mesostructure. (g) Poorly defined thrombolites leading to a mottled texture. (h) Stromatolitic mesostructure towards the top of the thrombolite. In all images v = vug, i = intraclasts, m = mesoclot, s = stromatolitic laminae, and g = grainstone.
Macrostructure of the Estheria Bed from Sedbury Cliffs: i = intraclast rudstone, g = grainstone, s = stromatolitic laminae, t = thrombolitic bioherms.
This thrombolitic microbialite layer in all samples is 2.0–4.5 cm thick (Figures 4 and 6). In cross section, the thrombolites appear as simple to polylobate and partially interconnected to anastomosed. They are composed of mid-brown-grey argillaceous limestone and are either mottled (mesoclots) (Figure 5c–f) or weakly laminated (Figure 5c, f, h). Thrombolites in the lower to middle parts of the microbialite have inter-thrombolite areas of light-grey argillaceous limestone, pelletal grainstone or calcite and geopetal-filled vugs (Figure 5b–h). The thrombolites are typically oriented horizontally near the base of the bioherm (Figure 5c–d), passing upward into more vertically orientated forms up to 5 mm in height (Figure 5c–d). The upper part of the layer comprises smaller, more irregular and more densely packed thrombolites and inter-thrombolitic sediment containing small shells (likely to be ostracods or Euestheria, which were recorded by Richardson, Reference Richardson1903 a at this level), elongate intraclasts, peloids and silt grains (Figure 5e, h). The irregular top is draped by sediment and locally by columnar stromatolitic laminae (Figures 4a and 5d, h). Mayall (Mayall, 1980, p. 181) examined thin sections of the bed and documented irregular dark micrite clots, up to 80 µm across, forming an open structure interspersed with calcite-filled irregular voids.
2.b. Cotham Marble microbialite
The Cotham Marble microbialite commonly comprises a bed of stromatolite bioherms 2–30 cm in height and up to one metre in diameter (Figure 7). Where exposure permits, as at Redland, Bristol [ST 576 751] and Stoke Gifford, Gloucestershire [ST 625 795], groups of microbialite bioherms can extend laterally for up to around 90 m (Short, Reference Short1903; Reynolds and Vaughan, Reference Reynolds and Vaughan1904). The Cotham Marble has been the subject of several detailed accounts that have attempted to classify the bed’s variation in morphology and structure and explain its origin (Woodward, Reference Woodward1892; Thompson, Reference Thompson1894; Short, Reference Short1903). Hamilton (Reference Hamilton1961) was the first to suggest a microbial origin.
Typical Cotham Marble from Stowey Quarry. (a) Polished section through a Cotham Marble bioherm. This displays hemispheroidal stromatolites draping mudcracks (1), low-amplitude hemispheroids (2), planar stromatolitic laminae (3), and dark grey, columnar branching thrombolites (4). They display laminated inter-thrombolite sediment that drapes the microbialites. Arrows=laminae highlighting synoptic relief of the thrombolites. Thrombolites are then capped by hemispheroidal stromatolites (5) that coalesce upwards. (b) Plan view of a typical bioherm of Cotham Marble. The ‘mammilated’ upper surface of smooth stromatolitic laminae is visible. In some of the depressions on the upper surface, the initiation of further stromatolite growth comprises small projections of columnar digitate stromatolites (arrowed).
Some workers have used the term ‘Cotham Marble’ interchangeably for the ‘Landscape Marble’ (microbialite) (Woodward, Reference Woodward1892; Thompson, Reference Thompson1894; Short, Reference Short1903; Hamilton, Reference Hamilton1961) and a mudflake breccia termed ‘Crazy Cotham Marble’ or ‘False Cotham Marble’ (Short, Reference Short1903; Hamilton, Reference Hamilton1961). The term ‘Landscape Marble’ was used to highlight branching features that resemble hedges and trees with laminated sediment draping the branching structures that were likened to a canopy (Figure 7a). Some of the variation observed in specimens was documented by Short (Reference Short1903), who described ‘double Cotham Marble’, ‘triple Cotham Marble’ (referring to three laminated layers rather than three cycles of laminated and dendritic microbialite) and a number of less typical forms such as ‘false Cotham Marble’ (synonymous with the ‘Crazy Cotham Marble’ of Hamilton, Reference Hamilton1961), mixed forms, spherical concretions and a simple laminated bed.
The Cotham Marble microbialite at Watchet, North Somerset, and Cribbs Causeway and Manor Farm, South Gloucestershire (Figure 3), contains the tubular fossil Microtubus in abundance (Wright and Mayall, Reference Wright, Mayall and Monty1981; Mayall and Wright, Reference Mayall and Wright2015; Figure 8). Microtubus microbialite clasts also occur within laterally equivalent conglomerates at Vallis Vale in the Mendip Hills, Somerset (Mayall, 1980) (Figure 3).
Cut and polished section of flake conglomerate from Manor Farm, Aust. The conglomerate lies at the same level as the Cotham Marble and contains fragments of laminated microbial limestone in a grainstone matrix comprising small carbonate grains and gastropods. The dark area represents the unoxidized centre of the limestone block. Several larger clasts are formed of Microtubus bioherm (arrowed) with a laminated stromatolite upper layer.
Various lithologies have been termed Cotham Marble in historic literature. In most instances, the name ‘landscape stone’ has been applied to specimens showing in situ arborescent growths (see Short, Reference Short1903). In our review of Cotham Marble occurrences in earlier literature, where there is ambiguity with the work of some authors, we only treat descriptions of the bed, including terms such as ‘landscape’ or ‘arborescent’, as potentially microbialitic (Figure 3). Mudflake breccias and the planar-laminated limestones (Short, Reference Short1903) are considered separately here, although we have seen specimens at Manor Farm Quarry where these and the microbialite facies are intermixed, and an example was figured by Hamilton (Reference Hamilton1961, plate 40) from Cotham.
Previous studies have provided comprehensive descriptions of the microbialite forms, including gross form, small-scale morphology and microstructure (Hamilton, Reference Hamilton1961; Wright and Mayall, Reference Wright, Mayall and Monty1981; Mayall and Wright Reference Mayall and Wright1981; Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014; Ibarra et al. Reference Ibarra, Marenco, Centlivre, Hedlund, Rademacher, Greene, Bottjer and Corsetti2024). Based on observations of specimens collected for our study, some aspects of the Cotham Marble are redescribed and reinterpreted.
2.b.1. ‘Dendritic’ facies
The Cotham Marble comprises a lower laminated stromatolite, with overlying branching structures (Figure 7a) commonly repeated in two layers (Figure 9a). The branching structures have variously been described as ‘dendritic arborescent structures’ (Wright and Mayall, Reference Wright, Mayall and Monty1981) and as a dendrolite (Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014; Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2016). In some examples, they dominate the mesostructure of the microbialite, and the laminated intervals are comparatively reduced. The description of the structures as a dendritic fabric or as a dendrolite does not conform to the definition of a dendrolite, which does not have a laminated internal fabric (Riding, Reference Riding and Riding1991; Suosaari et al. Reference Suosaari, Awramik, Reid, Stolz and Grey2018). In Cotham Marble specimens from Saleway and Manor Farm Quarry (Figure 3), the branching structures are small digitate to columnar branching stromatolites (Figure 9a–d). Where these columnar stromatolites are located at bioherm margins, they typically display greater height, fewer branches and are more closely clustered (Figure 9c). In some specimens from Saleway, they have been redeposited on top of the laminated stromatolite (Figure 9e). In specimens from Stowey Quarry (Figure 3), the internal mesostructure of the ‘dendritic’ facies is lost (Figure 7a), and at other sites (near Bristol) there is a mixture of laminated and structureless forms (Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014, Figure 5). This variable preservation of the digitate microbialites resembles the partly preserved stromatolitic laminae within the older Estheria Bed microbialite.
Macrostructure and mesostructure of the Cotham Marble from Saleway, Worcestershire: unless stated, scale bars = 2 mm. (a) Double landscape layer where the columnar branching stromatolites are seen well. (b) Close-up of columnar stromatolites seen in (a). (c) Edge of a small bioherm, displaying the closely clustered columnar stromatolites. (d) Bioherm with a thick laminated stromatolite middle layer. (e) Biostromal microbialite comprising a thin basal planar stromatolite, columnar branching stromatolites and a middle layer with fenestrae displaying a vertical fabric. The upper columnar stromatolites are rubbled. (f) Close-up view of the vertical fenestral fabric. (g) Upper surface of a biostrome 2.5 cm thick.
2.b.2. Tuft structures
Rare ‘algal’ tufts composed of tubular fenestrae were recognized in some specimens of the Cotham Marble (Mayall and Wright, Reference Mayall and Wright1981), who proposed that they represented pinnacle mats, formed by clumping of filaments of cyanobacteria. These tuft structures are also observed in one specimen from Saleway, Worcestershire. They are locally developed in place of the horizontal stromatolitic laminae (Figure 9e and f), grade upward from columnar branching stromatolites and overlain by an upper layer of columnar branching stromatolite fragments. The Worcestershire examples suggest that the features may be widespread but poorly documented within the Cotham Marble and representative of just one mat preservation style within the laminated part of the microbialite. The presence of other specimens without tufts from the same location may indicate that these variations were most likely on a small scale and result from variable microbial mat structures, different microbial communities or differences in rate of lithification.
2.b.3. Microtubus interactions
Enigmatic tubular microfossils assigned to Microtubus communis Flügel (generally considered to be a sessile polychaete worm tube) have been recorded within the Cotham Marble (Wright and Mayall, Reference Wright, Mayall and Monty1981) and in a few locations dominate the structure of the bioherm. The tubular microfossils are up to 2 mm long and straight to sinuous, with an internal diameter of 100–500 µm. They are either filled by spar or sediment and have micritic walls up to 80 µm thick (Wright and Mayall, Reference Wright, Mayall and Monty1981). The role of this microfossil in the accretion of the Cotham Marble has been debated (Wright and Mayall, Reference Wright, Mayall and Monty1981; Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014, Reference Ibarra, Corsetti, Greene and Bottjer2015; Mayall and Wright, Reference Mayall and Wright2015). Dendritic structures (columnar branching stromatolites) were interpreted as a symbiotic growth between the tube formers and the microbes responsible for the stromatolites (Wright and Mayall, Reference Wright, Mayall and Monty1981; Mayall and Wright, Reference Mayall and Wright2015). Other authors have suggested that growth of dendritic structures was not dependent on Microtubus and may instead have inhibited growth of the typical Cotham Marble, rather than promoting it (Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014, Reference Ibarra, Corsetti, Greene and Bottjer2015). Ibarra et al. (Reference Ibarra, Corsetti, Greene and Bottjer2014) favoured a gas bubble origin for many of the circular features seen in cross section that had been attributed to Microtubus, but through examination of specimens in this study, showing an absence of Microtubus in bioherms from Worcestershire (Figure 9) and its abundance in bioherms from Manor Farm Quarry (Figure 8), we do not agree with a gas bubble origin.
Comparison of specimens from Stowey Quarry, Manor Farm Quarry and Worcestershire indicates that the tube microfossils are an abundant component of the mesostructure of some, but not all, bioherms. Bioherms with abundant Microtubus are rare, however (Figure 3), and at Manor Farm (Figure 8) are a direct lateral equivalent of more typical ‘landscape’ Cotham Marble. Specimens of Cotham Marble from Stowey Quarry show some Microtubus fossils within the branching stromatolite part, but those from Worcestershire show none (Figure 9). Microtubus was therefore not essential in the formation of the Cotham Marble bioherms, as Mayall and Wright (Reference Mayall and Wright2015) suggested. The presence at Manor Farm Quarry of stromatolites both with and without abundant Microtubus suggests at times or in certain sub-environments, Microtubus flourished, leading to a different type of microbial structure (Figure 9).
2.c. Other microbialites within the Cotham Member
Other microbialites have been recorded in the Cotham Member and require further study. ‘Algal encrustations’ were documented by Poole and Whiteman (Reference Poole and Whiteman1966, p. 47) at outcrop and in two borehole cores in the Cheshire Basin (Figures 3 and 10), NW England, in the uppermost limestone bed of an undifferentiated Lilstock Formation. There are records of stromatolites at Lavernock, South Wales (Mayall, 1980), and in Warwickshire (English Midlands); laminated limestones with ‘algal structures’ were recorded from a railway cutting between Walton and Pillerton Priors by Edmonds et al. (Reference Edmonds, Poole and Wilson1965) (Figure 3). A thin stromatolite bed is seen towards the top of the Cotham Member in a cliff section at Cloghan Point in Northern Ireland (in the Larne Basin) (Raine et al. 2021; Jeram et al. Reference Jeram, Simms, Hesselbo and Raine2021) (Figure 3). They comprise small stromatolite bioherms (2–4 cm high and 5–15 cm in diameter). Intraclast rudstone occurs at the margins of the bioherms. The bioherms appear to encrust an irregular mudcracked surface, and laminae are marked by small fenestrae (Figure 11). Recently, Schinaia (S.A. Schinaia, unpub. MSc thesis, University of East Anglia, 2023) described a thrombolite bed immediately underlying the Cotham Marble from a location near Lower Woods, Gloucestershire [ST 745 877]. This bed bears a passing similarity to the Estheria Bed microbialite. Further study is required to ascertain its stratigraphic relations.
Structural and palaeogeographic setting of the U.K. and Ireland during the Rhaetian. The principal basins and known and postulated areas of land are shown in grey (modified after Ziegler, Reference Ziegler1990).
Stromatolite from the upper Cotham Marble of Cloghan Point, County Antrim, Northern Ireland. The stromatolite laminae drape what appears to be a mud-cracked mound of calcareous mudstone. Small calcite-filled fenestrae are seen (arrowed).
2.d. Flake breccias (‘crazy-’ or ‘false Cotham Marble’), planar-laminated limestone and other lithologies associated with the Cotham Marble
In the past, the term ‘Cotham Marble’ has included laminated or porcelaneous limestone and intraclast flake breccias, associated with landscape Cotham Marble. A laminated limestone at the same stratigraphic level has been recorded at several locations, e.g., Cossington (Somerset), Aust Cliff (Gloucestershire) and Lassington (Gloucestershire) (Woodward, Reference Woodward1892) (Figure 3). At Cossington and Aust Cliff, Cotham Marble is also recorded (Richardson, Reference Richardson1911; Short, Reference Short1904), suggesting that the laminated facies is a lateral equivalent of the stromatolite. Flake breccia has at times been referred to as ‘crazy-’ or ‘false Cotham Marble’ and is made up largely of laminated (both planar and microbial) limestone fragments (Figure 8). It occurs at Aust Cliff, Manor Farm, Aust and Redland [ST 576 751] and passes laterally into the Cotham Marble (Short, Reference Short1903; Radley and Carpenter, Reference Radley and Carpenter1998). Individual clasts may be angular or irregular and deformed, locally displaying microbial mat overgrowth, embedded in a matrix of grainstone or lime mudstone enclosing smaller intraclasts and small gastropods (Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014). Richardson (Reference Richardson1911) referred to a ‘Cotham Marble’ or ‘Cotham Marble equivalent’, comprising ‘splintery’, brown-grey porcellanous limestone without internal features (Richardson, Reference Richardson1911). This was suggested, alternatively, to be the basal bed of the overlying Langport Member (Whittaker, Reference Whittaker1978).
In north Gloucestershire and south Worcestershire, where the Cotham Marble appears to be absent, the ‘Monotis bed’ (Wright, Reference Wright1860) was taken to be at the same level by Etheridge (Reference Etheridge1865) and Richardson (Reference Richardson1903 c) (Figure 1). This bed was later renamed the ‘Pseudomonotis Bed’ (Richardson, Reference Richardson1904 a) after a small bivalve, now attributed to the genus Oxytoma. The presence of the bivalve Oxytoma fallax (Pflüker) along with Modiolus minimus (J. Sowerby) within the upper part of the Cotham Marble at Charfield and Wickwar, south Gloucestershire (Richardson, Reference Richardson1904 a), led Richardson to suggest that the bed is a time-equivalent of the Cotham Marble. The presence of these taxa in the overlying Langport Member elsewhere (Hallam, Reference Hallam1960) alternatively supports the assignment of the Pseudomonotis Bed to a higher stratigraphic level, above the Cotham Marble.
3. Discussion
3.a. Development of microbialites in the Cotham Member
Variation of growth forms within the Cotham Marble has been attributed to factors such as clumping of microbial filaments, colonization by tubular microfossils and changes in sedimentation and salinity (Wright and Mayall, Reference Wright, Mayall and Monty1981). Examination of Cotham Marble from several localities led Ibarra et al. (Reference Ibarra, Corsetti, Greene and Bottjer2015) to propose a common developmental history based on shared structural features. Examination of both the Estheria Bed and the Cotham Marble shows that the Estheria Bed developed in a similar way to the Cotham Marble or vice versa. The notable difference is that the Estheria Bed lacks much of the fine-grained laminated stromatolite limestone that characterizes the Cotham Marble. Vugs are more common, and grainstone is often associated with the microbialites. Thrombolites are dominant over stromatolites.
Fossil thrombolites have been variously interpreted as bioturbated stromatolites (Walter and Heys, Reference Walter and Heys1985; Burne and Moore, Reference Burne and Moore1987), primary skeletal structures (Kennard and James, Reference Kennard and James1986; Pratt and James, Reference Pratt and James1982; Walter, Reference Walter and Bengston1994) or a result of diagenetic processes (Planavsky and Ginsburg, Reference Planavsky and Ginsburg2009). Where the mode of formation is less certain or there is a mixture of clotted and laminated texture, the term microbialite with the qualifiers ‘thrombolitic’ and ‘stromatolitic’ (based on the mesostructure) has been used (Burne and Moore, Reference Burne and Moore1987; Planavsky and Ginsburg, Reference Planavsky and Ginsburg2009). Based on this, the Estheria Bed is a thrombolitic and stromatolitic microbialite.
3.b. Lagoonal setting of the Cotham Member
3.b.1 Environmental indicators and fossil evidence
Microbialites are distinctive indicators of environmental conditions, reflecting photic and nutrient requirements, salinity, energy levels, sedimentation rates, palaeotidal range and accommodation space. They have been widely used for correlation and palaeoenvironmental interpretation (Cloud and Semikhatov, Reference Cloud and Semikhatov1969; Hofmann, Reference Hofmann1973; Shapiro and Awramik, Reference Shapiro and Awramik2000).
Marine fossils are generally rare in the Cotham Member. In the Bristol Channel Basin and parts of the Worcester Basin (Figure 10), its basal beds contain marine microplankton, foraminifera and bivalves (Richardson, Reference Richardson1911; Hamilton, Reference Hamilton and Lord1982; Whittaker and Green, Reference Whittaker and Green1983; Old et al. Reference Old, Sumbler and Ambrose1987; Copestake, Reference Copestake1989; Bucefalo Palliani and Buratti, Reference Bucefalo Palliani and Buratti2006). This fauna resembles that of the underlying Westbury Formation (Warrington, Reference Warrington and Poole1977), and some bivalve specimens may be reworked from that unit (Ivimey-Cook, Reference Ivimey-Cook and Owen1974; Waters and Lawrence, Reference Waters and Lawrence1987). Beyond the Bristol Channel Basin and southern Worcester Basin, the Cotham Member is largely characterized by freshwater to brackish taxa. The aquatic liverwort Naiadita and algae are recorded in the ‘Naiadita Bed’ (equivalent to the Estheria Bed) (Brodie, Reference Brodie1865; Harris, Reference Harris1938; Knox, Reference Knox1982), and the branchiopod Euestheria, fish remains and the ostracod Darwinula are recorded in the Estheria Bed (Richardson, Reference Richardson1903 a). These taxa are recorded in the Cotham Member in the Midlands and farther east (Kent, Reference Kent1953; Poole et al. Reference Poole, Williams and Haines1968; Sykes et al. Reference Sykes, Cargill and Fryer1970; Powell et al. Reference Powell, Cooper and Benfield1992; Frost, Reference Frost1998). Only in some basins (Larne Basin in Northern Ireland) does there appear to have been continued marine connectivity (Opazo and Page, Reference Opazo and Page2021). In most basins, marine fossils are scarce and, where present, associated with the Cotham Marble (Short, Reference Short1904; Richardson, Reference Richardson1904 a; Duffin, Reference Duffin1980).
Sedimentary structures support deposition of the Cotham Member in very shallow water influenced by wave and microtidal currents with episodic storms. Flat-topped, low-amplitude symmetrical ripples (such as at Lavernock, South Wales) suggest a shallow, wave-dominated environment subject to exposure (Mayall, Reference Mayall1983, Hesselbo et al. Reference Hesselbo, Robinson and Surlyk2004). Recently, analysis of organic biomarkers collected through the member at St Audrie’s Bay and Lilstock, Somerset; Lavernock, South Wales; and in the Felixkirk Borehole, North Yorkshire [SE 4835 8576] suggests deposition within a non-marine, non-stratified, oxygenated, shallow water setting (Fox et al. Reference Fox, Cui, Whiteside, Olsen, Summons and Grice2020, Reference Fox, Whiteside, Olsen and Grice2022; Beith et al. Reference Beith, Fox, Marshall and Whiteside2021, Reference Beith, Fox, Marshall and Whiteside2023) consistent with deposition in a shallow brackish lagoon.
3.b.3. Depositional model and tectonic control
The scarcity of unequivocal marine fauna in the middle and upper Cotham Member has prompted varied and conflicting interpretations of its depositional environment. Proposed models included a shallow coastal carbonate ramp alternating between restricted and open-marine connection (Hesselbo et al. Reference Hesselbo, Robinson and Surlyk2004), a shoreface equivalent of the underlying transgressive Westbury Formation (Mander et al. Reference Mander, Twitchett and Benton2008), an inhospitable hypersaline tidal flat (Benton et al. Reference Benton, Cook and Turner2002), lacustrine (Kent, Reference Kent1970), shallow marine (Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014) and freshwater or brackish lagoon (Short, Reference Short1903; Mayall, Reference Mayall1983).
In the Bristol Channel Basin and Central Somerset Basin (St Audrie’s Bay, North Somerset, Lavernock Point, South Wales and Chilcompton, Somerset), the upper part of the Cotham Member overlies a regional erosion surface that displays deep syn-depositional cracks (likely to be desiccation-related). This horizon has been interpreted as evidence of eustatic sea-level fall (Hesselbo et al. Reference Hesselbo, Robinson and Surlyk2004, Wignall et al. Reference Wignall, He, Bond and Howard2025) or local tectonic uplift (Mayall, Reference Mayall1983). Soft-sediment deformation of strata below the erosion surface extends beyond the Bristol Channel Basin (Simms, Reference Simms2003, Reference Simms2007). A NE–SW-trending structure at the east end of the Bristol Channel – the lower Severn axis of Kellaway and Welch (Reference Kellaway and Welch1948) – may have experienced tectonic uplift resulting in the desiccation-cracked and eroded surface in that area (Ivimey-Cook, Reference Ivimey-Cook and Owen1974). The tectonism possibly relates to the proto-Atlantic opening (Swift, Reference Swift1999) or halokinesis (Trude et al. Reference Trude, Graham and Pilcher2012). Accumulation of sand- and ooid beds just above the deformed strata in the Bristol Channel Basin (Figure 10) at Lavernock, St Mary’s Well Bay (South Wales) and St Audrie’s Bay (north Somerset) (Mayall, Reference Mayall1983; Mayall, 1980) may be indicative of shoals that could have further isolated the lagoon from marine waters to the southwest in the Celtic Sea Basin (Figure 10). Above the desiccated surface, accommodation space in the lagoon would have gradually filled until subsidence or continued eustatic rise permitted increasingly intermittent marine flooding suggested by marine fossils associated with the Cotham Marble at the top of the member. This scenario could explain widespread microbial mat migration across the shallow waters of the whole lagoon. Despite the Cotham Marble having also been proposed as diachronous (Mayall and Wright, Reference Mayall and Wright2015), Ibarra et al. (Reference Ibarra, Corsetti, Greene and Bottjer2015) concluded that the consistent microstratigraphy of the Cotham Marble indicated deposition under regional environmental variation was the dominant control (see Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2014, Figure 2, Ibarra et al. Reference Ibarra, Marenco, Centlivre, Hedlund, Rademacher, Greene, Bottjer and Corsetti2024, Figure 6). This supports deposition within a single, extensive waterbody that experienced uniform fluctuations in depth, turbidity and salinity and resulting changes in microbial mat development. Restricted circulation and shallow waters likely promoted a microtidal regime (Kjerfve, Reference Kjerfve and Wolfe1986), facilitating the development of columnar stromatolites such as those well seen in the Cotham Marble at Saleway and Manor Farm Quarry. Two cycles of clay deposition are recorded in the lagoon, each culminating in microbialite growth.
3.c. Extent of the Cotham Member lagoon
A constricted waterway between Cornubia and the Welsh landmass may have provided a marine connection to the Celtic Sea Basin (Figure 10), explaining the presence of marine fauna in the basal beds of the Cotham Member within the Bristol Channel Basin (Waters and Lawrence, Reference Waters and Lawrence1987), resulting in a choked (single inlet) or restricted (two or more inlets) coastal lagoon (sensu Kjerfve, Reference Kjerfve and Wolfe1986).
As a minimum, the extent of the lagoon must be coincident with the extent of the Cotham Marble, which crops out for approximately 200 km from Devon, southwest England, to Warwickshire, English Midlands, giving an estimated aerial extent of at least 9,500 km2 (Figure 3). Modern lagoons of similar scale include the brackish Lagoa dos Patos, Brazil, which covers ∼10 227 km2 (Asmus, Reference Asmus1997). It is very likely, however, that the Cotham Lagoon extended far beyond this, as the facies and fauna along much of the remaining outcrop are remarkably uniform.
North-east of Warwickshire, similar facies but without microbialite characterize the Cotham Member as far as North Yorkshire (Cleveland Basin, Figure 10) (Frost, Reference Frost1998; Powell et al. Reference Powell, Cooper and Benfield1992; Wignall et al. Reference Wignall, He, Bond and Howard2025), ∼230 km beyond the last Cotham Marble occurrences in Warwickshire. In the Cleveland Basin, the member comprises soft grey-green calcareous mudstones with silt laminae, lacking macrofauna except Euestheria minuta (Alberti in Zieten). In the Cheshire and Larne basins (Figure 10), microbialites occur but are morphologically different to the Estheria Bed and Cotham Marble, and the correlation of beds to the main UK outcrop is more speculative and requires geochemical study.
3.d. Termination of lagoonal sedimentation
Following Duffin (Reference Duffin1980), Gallois (Reference Gallois2009) and others, we view the Cotham Member in southwest Britain (Bristol Channel Basin to Worcester Basin) as two mudstone-dominated packages that culminate in microbialite limestones (Figure 1). The top of each package is marked by a disconformity (Gallois Reference Gallois2009), indicating uplift and erosion in some cases. The specific conditions that led to the establishment and proliferation of microbialites over such a wide area on two separate occasions are not yet fully understood but are likely to relate to water depth changes in the lagoon, increased nutrient input, reduced sediment input or climatic change marking the top of each cycle and allowing establishment of microbial colonies and their calcification.
With the Estheria Bed locally developed near its top, the lower mudstone often preserves soft sediment deformation and locally demonstrates an erosive upper surface preserving deep mud cracks (Duffin, Reference Duffin1980; Mayall, Reference Mayall1983; Simms, Reference Simms2003, Reference Simms2007; Gallois, Reference Gallois2009). The upper mudstone cycle usually culminates in the Cotham Marble and is overlain in most places by the Langport Member, apparently conformably. However, in the northern part of the Bristol–Mendip archipelago (Lovegrove et al. Reference Lovegrove, Newell, Whiteside and Benton2021), north of Bristol and into South Gloucestershire, the Langport Member is absent at Lower Woods (Richardson, Reference Richardson1906 b), Manor Farm Quarry and Aust Cliff (Radley and Carpenter, Reference Radley and Carpenter1998). Sometimes the Cotham Marble is also absent (as at Sedbury Cliffs, where a conglomerate is observed marking the base of the Blue Lias Formation, Richardson, Reference Richardson1903 b). It is not known if the erosion was later at the base of the Lias Group or at the time equivalent of the Langport Member elsewhere. The conglomerate at Sedbury Cliffs, however, contains clasts similar to the Langport Member in a matrix typical of the Blue Lias Formation (Swift, Reference Swift1995), suggesting the erosion was a later event.
In a number of basins in southwest Britain (Wessex, Central Somerset and Bristol Channel basins) and in Northern Ireland (Larne and Lough Foyle basins), the upward transition from the Cotham Member to the Langport Member is more conformable (Gallois, Reference Gallois2007; Gallois, Reference Gallois2009, Raine et al., 2021). At some locations where the Langport Member is present, it has sometimes preserved evidence of erosion shortly after deposition of the Cotham Member, possibly one of the causes for the absence of Cotham Marble at many localities north of South Gloucestershire. However, localized erosion or removal of the Cotham Marble prior to or at the time of initial Langport Member sedimentation is indicated by bored, encrusted and reworked fragments of the bed from several basins. In the Central Somerset Basin, bored (Gastrochaenolites) and Atreta-encrusted nodules of Cotham Marble occur in situ at Compton Hill, Compton Dundon (Richardson, Reference Richardson1911). Further north in the Worcester Basin at Saleway, where the Langport is represented by a single limestone bioturbated by Diplocraterion, the underlying Cotham Marble has commonly been bored (Gastrochaenolites), encrusted by Atreta and in some instances subsequently eroded into clasts (Figure 12). On the East Midlands Shelf, at Southam Cement Works (Warwickshire), the base of the Langport Member is marked by a bed rich in bivalves (Atreta intusstriata (Emmrich)) and bioencrusted limestone pebbles, including recognizable Cotham Marble fragments displaying regular echinoid grazing traces (Radley, Reference Radley2002). These sections show that the Cotham Marble formed a marine hardground following the flooding and demise of the Cotham Member Lagoon.
Pieces of bioeroded and encrusted Cotham Marble from Saleway. (a–b) Gastrochaenolites borings. (c) Atreta encrusted surface, with scattered shallow vertical borings. (d) Solution-planed surface on which there are raised attachment sites of Atreta. The majority of the shells have been eroded off the surface.
3.e. Microbialites, extinction and environmental change in the Cotham Member
Mander et al. (Reference Mander, Twitchett and Benton2008) interpreted the Cotham Member as a ‘dead zone’ linked to the end-Triassic mass extinction. Synchroneity of the Cotham Marble with the extinction horizon was proposed by Ibarra et al. (Reference Ibarra, Corsetti, Greene and Bottjer2014, Reference Ibarra, Corsetti, Greene and Bottjer2016), based on its lateral extent, the assumed coincident major negative δ1 3C excursion and the occurrence of the prasinophyte green alga Tasmanites, which suggests strong environmental instability and is recorded from other sections of this interval (van de Schootbrugge et al. Reference van de Schootbrugge, van der Weijst, Hollaar, Vecoli, Strother, Kuhlmannd, Thein, Visscher, van Konijnenburg-van Cittert, Schobben, Sluijs and Lindström2020). Ibarra et al. (Reference Ibarra, Corsetti, Greene and Bottjer2014, Reference Ibarra, Corsetti, Greene and Bottjer2016) further suggested that microbialite development resulted from reduced grazing pressure following a post-extinction decline in metazoan diversity.
The relationship between microbialite abundance and metazoan influence has long been recognized, with the sharp decline in microbialite abundance at the end of the Proterozoic attributed to grazing associated with increasing metazoan diversity (Awramik, Reference Awramik1971; Walter and Heys, Reference Walter and Heys1985). Conversely, microbialites proliferated after major extinction events, notably at the end-Ordovician (Sheehan and Harris, Reference Sheehan and Harris2004) and end-Permian (Schubert and Bottjer, Reference Schubert and Bottjer1992; Mary and Woods, Reference Mary and Woods2008), arguably due to reduction in grazing pressure.
The Cotham Member environment was likely characterized by reduced salinity (see above), excluding many metazoan taxa, meaning that the preservation of the Cotham Marble is not necessarily related to an absence of grazers or infauna, and the interval masks extinction events that may have been occurring in marine environments at this time, postulated by Wignall and Atkinson (Reference Wignall and Atkinson2020).
The Cotham Marble coincides with the peak of a negative carbon isotope excursion, termed the ‘initial’ CIE (Hesselbo et al. Reference Hesselbo, Robinson, Surlyk and Piasecki2002, Reference Hesselbo, Robinson and Surlyk2004; Ibarra et al. Reference Ibarra, Corsetti, Greene and Bottjer2016). This excursion has been used to correlate Late Triassic records with the Global Boundary Stratotype Section and Point (GSSP) for the base of the Jurassic. However, Fox et al. (Reference Fox, Cui, Whiteside, Olsen, Summons and Grice2020) questioned its utility as a global marker, demonstrating that at its type locality (St Audrie’s Bay) in the Bristol Channel Basin, the initial carbon isotope excursion (CIE) reflects a marine-to-nonmarine transition caused by abrupt relative sea-level fall. Biomarker and compound-specific isotopic data (Fox et al. Reference Fox, Cui, Whiteside, Olsen, Summons and Grice2020, Reference Fox, Whiteside, Olsen and Grice2022) indicated that microbial mat proliferation, driven by fresh-to-brackish water influx, supplied isotopically light carbon to both organic and inorganic pools in very shallow waters and is a possible explanation for the negative excursion. Thus, the CIE and associated marine biotic decline at this locality likely reflect local environmental change rather than a global extinction or atmospheric carbon perturbation.
Wignall and Atkinson (Reference Wignall and Atkinson2020) suggested two extinction pulses: one at the top of the Westbury Formation/base of the Cotham Member and another at the top of the Langport Member. They suggested that the lower event occurred within the brackish-water deposits of the upper Cotham Member, its signal obscured by environmental variability. In sections with better marine records, such as the Larne Basin, data indicate modest extinction – primarily among bivalves – through the lower Cotham Member to the lower Langport Member interval (Opazo and Page, Reference Opazo and Page2021).
4. Conclusions
This study demonstrates that the Cotham Member contains two distinct, regionally developed microbialite horizons – the well-known Cotham Marble (stromatolitic) and the older, previously underdescribed Estheria Bed (stromatolitic and thrombolitic). Together these horizons reveal a more complex and environmentally sensitive microbial ecosystem than previously recognized. The new field observations considerably expand the known distribution of both microbialites, showing that they occur across multiple basins in southern Britain and possibly Northern Ireland and are not restricted to the classic sections of southwest England. This wider distribution supports their interpretation as coeval deposits formed under uniform, lagoonal conditions. We envisage a single, large lagoon across the Wessex Basin, Bristol Channel Basin, Worcester Basin and onto the East Midlands Shelf (Figure 10), due to the uniformity of the features within the Cotham Marble across a wide area (9500 km2) and the uniformity of the Estheria Bed over a smaller area of 4500 km2.
During lagoonal deposition, reduced salinity likely excluded metazoan grazers, enabling microbialite proliferation and masking faunal extinction signals. The Cotham Marble coincides with a negative carbon isotope excursion previously linked to the end-Triassic extinction, but biomarker evidence suggests this reflects local environmental change rather than global carbon-cycle perturbation.
Rather than growth of a dendrolite, the dendritic or arborescent facies of the Cotham Marble represent growth of a branching, columnar stromatolite, initiated by horizontally branching borings and/or burrows. The precise relationship between the borings and/or burrows and Microtubus is not known, although they are of a similar diameter. In some cases, Microtubus appears to form an important part of the structure. The dendritic features in the Estheria Bed are more thrombolitic, but whether these are also the product of alteration of stromatolites requires petrographic analysis and a more detailed study.
Finally, the repeated development of microbialites at the tops of two mudstone packages underscores the importance of water depth fluctuations, sediment supply and episodic ecological windows in promoting widespread microbial colonization. The Estheria Bed and Cotham Marble are key indicators of lagoonal evolution and offer improved correlation potential of the Cotham Member across multiple basins and a refined understanding of the environmental dynamics operating at this critical time.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756826100958.
Acknowledgements
Gareth Farr is thanked for field assistance at Manor Farm, and the owner of Manor Farm is thanked for permission to visit Manor Farm Quarry in 2019. Peter Hodges and Mike Simms are thanked for providing comments on an early version of the manuscript. Julian Andrews and Stefania Schinaia are thanked for their useful discussion on the topic and comments on the manuscript. This research received no specific grant from any funding agency, commercial or not-for-profit sectors. We thank two anonymous reviewers for their valuable comments and suggestions that improved this manuscript.
Competing interests
The authors declare none.
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